an investigation of stepped open slot antenna with ... · c1 = lwε0ε e 2h cos2 πx 0 l (1) r1 = q...

Progress In Electromagnetics Research C, Vol. 91, 157–171, 2019

An Investigation of Stepped Open Slot Antenna with Circular

Tuning Stub

Prashant Purohit*, Bhupendra K. Shukla, and Deepak K. Raghuvanshi

Abstract—In this article, a microstrip-fed stepped open slot antenna is presented which is suitablefor GSM 1800, WiFi, WiMAX, PCS, and ITM-2000 applications. The proposed geometry is composedof a circle-shaped tuning stub, a feed structure, and deformed ground plane. The proper tuning ofresonating modes (fr1, fr2, fr3 and fr4) and wideband frequency response are acquired by adjustingthe dimension of stairs, tuning the stub and an elliptical slot. The experimental result demonstratesthat this antenna covers the frequency range from 1.375 to 5.6 GHz with measured fractional bandwidth(BW (%) = 200 ∗ (fh − fl)/(fh +fl)) of 121.14% for S11 < −10 dB. This antenna also exhibits resonanceat frequencies (measured) 1.625, 2.52, 2.82, 3.75, 4.67, and 5.42 GHz. After investigating the surfacecurrent distribution, the mathematical equations are deduced for simulated resonating frequencies of1.35, 2, 3.8, and 5.22 GHz. Due to asymmetry in structure, asymmetric far-field patterns are found inE-plane with omnidirectional patterns in H-plane.

1. INTRODUCTION

The broadband feature of a planar antenna depends on the overlapping of resonating modes which canbe achieved by choosing a proper dimension of the tuning stub and other elements of the antenna [1].Many reported configurations, such as wide slot antenna [2–4], monopole antenna [5], and CPWfed monopole [6], are famous in the wireless industry due to their attractive features, such as wideimpedance bandwidth, low profile, low cost, and easy integration with microwave circuitry. Due to halfwavelength resonance, the size of above-mentioned antennas is large which is their main demerit. In thepresent scenario, the requirement of a compact antenna with wide impedance bandwidth is increasing.Printed open slot antenna is a potential candidate which offers compactness due to quarter wavelengthresonance [7]. In addition, open slot antenna exhibits the ability to produce circular polarization andasymmetric radiation pattern due to its asymmetric geometry [8, 9]. The role of slots is prominentin above-mentioned antennas. Indeed, slots modify the path length of surface current vector whichchanges the position of resonating frequency. It also alters the effective capacitance and inductance ofthe antenna which directly affects the phase velocity (vp = 1/

√LC) of the resonating frequency [10–12].

Loading of the slot on the ground plane and patch provides new edges for fringing and also producesnew resonating frequencies [13]. The impedance bandwidth of the open slot antenna can be modified bythe following methods 1) By introducing notches on the circumference of the open slot which changesthe resonant path length [7], 2) By cutting the steps on the open slot [14] which changes the mutualcoupling between ground plane and radiating element, 3) By tapering the slot [15, 16]. The rotation ofthe radiating patch produces a large number of resonating modes, and by choosing the proper angle,these modes can be overlapped [17]. Some other methods are reported for enhancing the bandwidthof open slot antenna, such as integrating tuning stub [18], changing the shape of feed structure, andradiating element [19, 20].

Received 14 February 2019, Accepted 14 March 2019, Scheduled 24 March 2019* Corresponding author: Prashant Purohit ([email protected]).The authors are with the Maulana Azad National Institute of Technology, Bhopal, India.

158 Purohit, Shukla, Raghuvanshi

In this communication, we design a stepped open slot antenna with a circular tuning element whichis suitable for applications like GSM 1800, WiMAX, PCS and ITM-2000. The wide frequency responseof this antenna is achieved by adjusting the dimension of the tuning stub, stair, and elliptical slot. Itoccupies the fractional bandwidth of 121.14% for S11 < −10 dB that covers the frequency span from1.375 to 5.6 GHz. The circuit model of the proposed antenna is also presented. At frequencies 1.35 GHz,2GHz, 3.8 GHz, and 5.22 GHz, the surface current distribution is analyzed, and the mathematicalequations are deduced.

2. ANTENNA CONFIGURATION

The physical structure and parameters of the proposed antenna are depicted in Figure 1. It contains slotloaded ground plane, a feed structure, and a circular tuning stub. This antenna is placed on XY = 0plane and fabricated on an FR-4 substrate with the dimension of 66mm × 45 mm. The properties ofthe chosen substrate are tangent loss tan(δ)= 0.02, permittivity εr= 4.3, and height h = 1.6 mm. Thethickness of the conducting layer is taken 0.035 mm. The circular tuning stub with radius Rt and feedstructure (Fw × Fl) are designed on the top face of the substrate. In numerical analysis, this structureis excited by waveguide port. In the ground plane, an elliptical slot with parameters Ry (semi minoraxis radius) and Rx (semi major axis radius) is etched which improves the impedance matching of theantenna in the interested frequency band. Further, stairs of uneven size are introduced which changesthe location of resonating frequencies and impedance bandwidth of the antenna. The parameters ofthe stairs are Sl1, Sw1 (first step), Sl2, Sw2 (second step), and Sl3, Sw3 (third step). To enhance theimpedance bandwidth of the antenna, a rectangular shaped slot with size 23mm × 20 mm is truncatedon the left bottom side of the ground plane. Table 1 shows the dimension of the proposed antenna.

Figure 1. Configuration of the propose stepped open slot antenna with circular tuning element.

3. EVOLUTION OF PROPOSED STEPPED OPEN SLOT ANTENNA

The development of the proposed stepped open slot antenna is displayed in Figure 2. It is noticed thatthe shape of the ground plane is customized in succeeding stages. Figure 3 displays the comparisonof reflection coefficient characteristics of the antennas, and performances of all antennas are listed inTable 2. Antenna 1 comprises an inverted L-shaped slot in the ground plane and a circular tuning stub.It shows dual-band characteristic with two resonating frequencies (see Table 2). By loading the slot, the

Progress In Electromagnetics Research C, Vol. 91, 2019 159

Table 1. The dimension of the stepped open slot antenna with circular tuning element.

ParameterDimension

(mm)Parameter

Dimension

(mm)Parameter

Dimension

(mm)Parameter

Dimension

(mm)

Fw 3 Ws 45 Ry 7 Sl2 4

Fl 19 L1 43 W2 7.77 Sw2 5

Rt 9 L2 31 W3 2.77 Sl3 6

Rx 8 L3 15 Sl1 10 Sw3 5

Ls 66 W1 20 Sw1 5

Table 2. Performance of antenna 1, 2, 3, 4 and 5.

Name ofAntenna

Response BandBandwidth

(%)Resonance frequency

(GHz)

A1 Dual 1.67 to 1.87 GHz 11.29 1.77, 5.425.00 to 6.00 GHz 18.18

A2 Dual 1.65 to 1.92 GHz 15.12 1.77, 3.4752.77 to 4.45 GHz 46.53

A3 Dual 1.35 to 2.25 GHz 50 1.75, 4.1252.62 to 4.42 GHz 51.13

A4 Single 1.35 to 5.60 GHz 122.30 1.47, 1.95, 3.87, 5.2A5 Single 1.27 to 5.62 GHz 126.27 1.35, 2, 3.8, 5.22

Figure 2. Schematic of evolution of the stepped open slot antenna with circular element.

effective inductance and capacitance of the antenna can be changed which modifies the bandwidth andposition of resonating frequencies [8]. In the next step (antenna 2), an elliptical slot is introduced onthe circumference of the inverted L-shaped slot. Due to this modification, impedance matching is raisedin lower and mid frequency bands while resonance frequency at 5.42 GHz has disappeared. Further,the stair (antenna 3) is created on the ground plane that upgrades the coupling between slot andtuning stub. To achieve wideband response and upgrade the capacitive coupling between the slot andtuning stub, another step of the stair (antenna 4) is introduced which refines the impedance matchingthroughout the frequency band. Antenna 4 shows the impedance bandwidth of 122.30% from 1.35 to5.6 GHz for S11 < −10 dB with four resonating frequencies. To modify the position of lower and highercutoff frequencies, a rectangle-shaped slot is truncated on the left bottom side of the ground plane.Antenna 5 covers a frequency band from 1.27 to 5.62 GHz and exhibits the bandwidth of 126.27% forS11 < −10 dB.


Figure 3. Comparison of reflection coefficient characteristics of antenna 1, 2, 3, 4, and 5.

4. CIRCUIT MODEL OF PROPOSED STEPPED OPEN SLOT ANTENNA

The circuit model of probe-fed patch antenna is shown in Figure 4. It is the parallel combination ofinductance (L1), capacitance (C1), and resistance (R1) [21–23].

Figure 4. Circuit model of the rectangular patch antenna.

The values of L1, C1, and R1 can be calculated by the following equations [21–23].

C1 =LWε0εe

2hcos2

(πx0

L

)(1)

R1 =Q

ω2rC1

(2)

L1 =1

C1ω2r

(3)

Q =c√

εe

4fh(4)

where L and W are the length and width of the patch. x0 and εe are the feed location and effectivepermittivity of the substrate. Q and h are the quality factor and height of the substrate, respectively.


The input impedance of the rectangular patch antenna is given by

Zin = jωLprobe + 1/(

1R1

+1

jωL1

+ jωC1

)(5)

The equivalent circuit of the proposed antenna is depicted in Figure 5 which is the series connectionof the parallel RLC circuit. The broad frequency response of this circuit model is acquired after theoverlapping of a large number of the resonating modes. The position of resonating frequency dependson the values of Li, Ci, and Ri, where i varies from 1 to 10. This circuit is modeled in two steps. Inthe first step, the values of R, L, and C are obtained from simulated reflection coefficient characteristic(using CST) at resonating frequencies. In the second step, the circuit is implemented in ADS, and thesevalues are tuned to achieve a proper response. The total impedance of the proposed circuit model canbe computed by equation below.

Zin =10∑i=1

1/(

1Ri

+1

jωLi

+ jωCi

)(6)

Figure 5. Circuit model of proposed stepped open slot antenna with circular tuning stub.

Table 3 represents the obtained values of R, L, and C from ADS. The comparison of reflectioncoefficient characteristics is shown in Figure 6. The red curve is the frequency response of the proposedcircuit model.

5. TUNING OF PARAMETERS

5.1. Influence of Radius (Rt) of Circular Tuning Stub

The simulated reflection coefficient characteristic vs frequency for different values of Rt is depicted inFigure 7. By increasing Rt, the overlapping area increases with an open slot which affects the capacitivecoupling between the slot and tuning stub. It is observed that this parameter critically affects theimpedance bandwidth of the antenna. By increasing Rt up to 9 mm, the impedance matching in thelower and mid frequency bands is improved. For Rt > 9mm, the bandwidth of the antenna is decreaseddue to over capacitive coupling.

5.2. Impact of Parameter of Stair (Sl3 and Sw3)

The effect of parameters of stair on reflection coefficient characteristic is displayed in Figure 8. Thelength of the stair (Sl3) mainly affects the impedance matching in the lower frequency band. With


Figure 6. Comparison of frequency response obtained from CST and circuit simulation.

Table 3. Obtained value of R, L, C through CST and ADS.

Obtained Values from ADSResistance (Ω) Capacitance (pF) Inductance (nH)R1 25.115 C1 27.36 L1 0.32R2 44.083 C2 9.422 L2 1.924R3 40.68 C3 25.02 L3 0.555R4 28.008 C4 11.455 L4 0.1384R5 26.935 C5 5.4 L5 0.99R6 47.421 C6 26.811 L6 0.029R7 63.276 C7 3.289 L7 0.87R8 41.93 C8 2.25 L8 0.4016R9 54.155 C9 4.284 L9 0.1056R10 31.98 C10 0.414 L10 0.4895

Obtained Values from CSTResistance (Ω) Capacitance (pF) Inductance (nH)R1 50.23 C1 34.20 L1 0.40R2 33.91 C2 6.73 L2 1.48R3 67.80 C3 16.68 L3 0.37R4 31.12 C4 22.91 L4 0.173R5 53.87 C5 3.60 L5 0.66R6 52.69 C6 29.79 L6 0.058R7 52.73 C7 2.53 L7 0.58R8 59.90 C8 2.26 L8 0.502R9 54.155 C9 7.14 L9 0.132R10 53.30 C10 0.828 L10 0.979


Figure 7. Simulated S11 versus frequency for different values of Rt of the proposed antenna.

Figure 8. Simulated S11 versus frequency for different lengths and widths of stair.

increasing Sl3, the location of the second resonating frequency (fr2) is shifted towards lower frequencyband while frequencies fr1 and fr3 do not change their position. The width of the stair (Sw3) changesthe position of the higher cutoff frequency (fh) and bandwidth of the antenna. With increasing Sw3,the location of fh is shifted towards lower frequency band. Due to this, the bandwidth of the antennais reduced. The frequencies fr2 and fr4 are shifted towards lower frequency band. It is observed thatthe uneven impedance matching is found at fr1 and fr3.

5.3. Influence of Radius (Rx and Ry) of the Elliptical Slot

The effect of Rx (major axis radius) and Ry (minor axis radius) on reflection coefficient characteristicof the proposed antenna is depicted in Figure 9. These two parameters affect the overlapping area of


Figure 9. Simulated S11 versus frequency for different radii of elliptical slot.

open slot with tuning stub. It can be noticed that by the major axis radius only affects the impedancematching at frequencies fr1, fr2, fr3, and fr4. The resonance frequency fr2 is drifted toward lowerfrequency band with increasing the length of the major axis radius. The minor axis radius Ry criticallyaffects the position of higher cutoff frequency and bandwidth of the antenna. With increasing Ry, theimpedance matching is improved at lower frequency band while the resonating frequencies fr4 and fr5

are shifted in leftward direction.

6. EXPERIMENTAL RESULTS AND DISCUSSION

6.1. Reflection Coefficient Characteristic

The fabricated stepped open slot antenna with a circular tuning stub is shown in Figure 10. Thesimulated results and optimization of the antenna are achieved by CST Microwave Studio. The comparedreflection coefficient characteristics of proposed antenna are shown in Figure 11. The measured results ofinput impedance and reflection coefficient are acquired from Agilent Technologies based Vector NetworkAnalyzer N2223A in frequency range 1 to 6 GHz. The proposed antenna exhibits the measured fractionalbandwidth of 121.14% for S11 < −10 dB which covers the frequency range from 1.375 to 5.6 GHz. Thisantenna also exhibits the resonance at frequencies (measured) 1.625, 2.52, 2.82, 3.75, 4.67, and 5.42 GHz.The simulated resonance is found at frequencies 1.375, 2, 3.8, and 5.22 GHz.

A small error of 7.2% between simulated and measured lower cutoff frequencies is estimated. Itis noticed that the positions of measured and simulated resonating frequencies are not matched due tofollowing reasons. 1) Connector and conductor loss, 2) Variation of dielectric constant. An error of0.89% is also computed between measured and simulated higher cutoff frequencies. It is analyzed thatthe lower cutoff frequency (flc) of the proposed antenna is controlled by the dimension of the groundplane. The frequency formulation of flc is given below [11, 12].

P1 = L1 + W1 + (Ls − L1) + (Ws − W1) (7)P1 = Ls + Ws (8)

flc =c

P1√

εr

(9)

where P1 is the path length, and c and εr are the speed of light and permittivity of the substraterespectively. The calculated value of path length P1 is 111 mm. By using Equation (9), the computed


Figure 10. Configuration of fabricate steppedopen slot antenna with circular tuning element.

Figure 11. The compared reflection coefficientcharacteristic versus frequency plot.

value of flc is 1.29 GHz which is nearly equal to measured and simulated lower cutoff frequencies. Anerror of 1.16% is found between simulated and calculated flc.

6.2. Input Impedance

The compared input impedance of the proposed stepped open slot antenna is depicted in Figure 12. Itis noticed that multiple loops are found inside of the VSWR circle. These loops indicate the mutualcoupling and overlapping between resonating modes which are required for realization of wide impedancebandwidth.

Figure 12. Compared input impedance characteristic of the proposed stepped open slot antenna.


6.3. Current Distribution

At simulated resonating frequencies, the surface current distribution of proposed antenna is investigatedwhich is depicted in Figure 13. After excitation, the current vectors are scattered on tuning stub and slotloaded ground plane. At frequency 1.35 GHz, the current vectors sink towards right bottom corner ofthe ground plane, and maximum intensity of current is investigated near elliptical slot. This resonatingfrequency is developed due to stepped open slot and side edge of the ground plane. The frequencyformulation of the first resonating frequency is given below [11, 12].

P2 = Sl1 + Sw1 + Sl2 + Sw2 + Sl3 + Sw3 + L3 + W3 + L2 + W2 + E (10)

E = 0.7 ∗ π ∗√

(R2x + R2

y)/2 (11)

f1 =c

P2√

εr

(12)

where P2 is the path length, and εr is the dielectric constant. The estimated value of P2 is 108.02 mm.By using Equation (12), the calculated value of f1 is 1.328 GHz. An error of 3.41% is estimated betweenthe first simulated and calculated resonating frequencies. At frequency 2GHz, the current vectors sinktowards right top corner of the ground plane, and circulation of current vectors is also investigated onthe tuning stub. This frequency is developed due to stepped open slot, and frequency formulation of

Figure 13. The simulated surface current distribution at frequency 1.35, 2, 3.8, and 5.22 GHz.


this second resonating frequency (f2) is given below.

P3 = Sl1 + Sw1 + Sl2 + Sw2 + Sl3 + Sw3 + L3 + W3 + W2 + E (13)

f2 =c

P3√

εr

(14)

where P3 is the path length, and the calculated value of P3 is 77.02 mm. By using Equation (14),the calculated value of f2 is 1.87 GHz which is nearly equal to 2GHz. An error of 6.5% is foundbetween simulated and computed second resonating frequencies. The resonance frequency 3.8 GHz (f3)is generated due to open slot, and this frequency is estimated by following equations.

P4 = L3 + W3 + (W2/2) + E + (Sw3/2) (15)

f3 =c

P4√

εr

(16)

where P4 is the path length, and the calculated value of P4 is 40.675 mm. The calculated value of f3

is 3.52 GHz. An error of 7.36% is found between simulated and computed third resonating frequencies.The fourth resonating frequency 5.22 GHz is developed due to the circular tuning element, and it isnoticed that the current vectors are distributed like TM10 mode on this element.

P5 = π ∗ Rt (17)

f4 =c

P5√

εr

(18)

where P5 is the half perimeter of circular tuning element, and the calculated value of P5 is 28.26 mm.The calculated value of f4 is 5.1 GHz which is nearly equal to 5.22 GHz.

6.4. Far Field Pattern

The comparison between simulated and measured radiation patterns at frequencies 1.35, 2, 3.8, and5.22 GHz is displayed in Figure 14. We have noticed the asymmetric radiation (in E plane) patternwhich is found due to asymmetry structure. At frequency 1.35 GHz, the pattern is omnidirectionalin H plane while asymmetric pattern is found in E plane. The shape of the pattern is changed dueto existence of higher order mode with fundamental mode. At frequencies 2, 3.8, and 5.22 GHz, theshape of the omnidirectional pattern is changed. The simulated patterns in E plane are almost similarin all resonating frequencies. Measured patterns slightly differ from the simulated ones because ofmultiple reasons: 1) radiation from the coaxial cable at lower frequency, 2) environment surroundingthe measuring instruments, 3) measurement and fabrication error. Figure 15 exhibits the graph ofsimulated gain and efficiency versus frequency. The realized gain varies between 2.1 and 4.1 dBi. It is

(a)


(b)

(c)

(d)

Figure 14. H plane (left) and E plane (right) pattern at frequencies (a) f1 = 1.35 GHz, (b) f2 = 2 GHz,(c) f3 = 3.8 GHz and (d) f4 = 5.22 GHz.


Figure 15. The simulated realized gain and efficiency of the proposed antenna.

Table 4. Comparison of proposed antenna with existing open slot antenna.

Reference Dimensions (mm2) Bandwidth (%) Substrate Frequency range (GHz)[7] 30 × 35 130.30 FR4 2.3–10.9[10] 9.2 × 9.8 48 RT5880LZ 5–8.17[17] 25 × 25 117.5 FR4 2.88–11.08

[19] 40 × 20 - FR4

1.54–1.612.31–2.723.10–3.755.03–5.95

[18] 30 × 35 131 FR4 2.3–11.1Proposed 66 × 45 121.14 FR4 1.375–5.6

investigated that the efficiency (Radiation and total efficiency) of the antenna is declined as operatingfrequency increases, which is because of dielectric loss. The comparison of proposed antenna withexisting open slot antennas in terms of size and bandwidth is shown in Table 4. All these antennasare designed on an FR-4 and RT5880LZ substrate. The proposed antenna exhibits good impedancebandwidth. To cover lower frequency band, the size of proposed antenna is larger than other.

7. CONCLUSION

The stepped open slot antenna with a circular tuning element is simulated, fabricated, and tested withthe help of Vector Network Analyzer N2223A in frequency range 1 to 6GHz. After investigation, it isnoticed that the step size of stair, radius of tuning stub, and major axis radius of elliptical slot criticallyaffect the impedance matching and bandwidth of the antenna. The proposed antenna exhibits themeasured fractional bandwidth of 121.14% for S11 < −10 dB which covers the frequency range from1.375 to 5.6 GHz. This antenna also exhibits the resonance at frequencies (measured) 1.625, 2.52, 2.82,3.75, 4.67, and 5.42 GHz. At the best matching frequencies 1.35, 2, 3.8, and 5.22 GHz, the currentdistribution is analyzed, and series of equation are deduced. The simulated and measured radiationpatterns are compared in both planes. The omni-directionality is lost due to higher order modes, andasymmetry pattern is found due to the structure.


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an investigation of stepped open slot antenna with ... · c1 = lwε0ε e 2h cos2 πx 0 l (1) r1 = q...

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